1. Field of the Invention
The present invention relates generally to the art of solar energy collection and more particularly to the art of producing an efficient solar energy receiving surface by wet chemical deposition of a selectively absorbing film.
2. Description of the Prior Art
A large proportion of the solar energy reaching the surface of the earth is in the wavelength range of visible light. It is well known that, in general, light bodies are good reflectors while dark bodies are good absorbers of visible light. For this reason, it has long been a practice in the art of solar energy collection to blacken the surface which receives solar radiation. However, it is also known that dark bodies may be strong radiators of heat. Thus, for the most efficient collection and utilization of solar energy, a surface is needed which is both an excellent absorber of energy in the form of visible light, and a poor radiator of energy in the form of heat; i.e., a selective surface is one which is black with respect to absorption of visible light and white with respect to radiation of heat.
It is another object of the present invention to provide a surface which is a good absorber of incident solar energy but which is not a strong radiator of heat.
It is known in the art to deposit a thin dark metal surface layer on a bright metal base to achieve the aformentioned desirable combination of properties. A well-known method for making such a selective composite surface is disclosed in U.S. Pat. No. 2,917,817 to Tabor. The method involves immersing the metal base as a cathode into an aqueous electrolytical bath of nickel sulfate, zinc sulfate, ammonium sulfate, ammonium thiocyanate and citric acid. By electrodeposition, a black coating is deposited on the base. The composite solar energy receiver thus produced has a high solar energy absorptance and a low thermal emissivity when compared with an ideal black body.
However, it is a further object of this invention to provide a selective solar energy absorbing surface by a method which is rapid, simple, and inexpensive, not requiring electroplating equipment.
Wet chemical methods are well-known in the art for producing coatings of metals, metal oxides and metal sulfides, some of which are black. U.S. Pat. No. 3,314,811 to Mitchell et al. discloses a composition for blackening aluminum at room temperature by conventional immersion techniques which do not require applied current. The composition disclosed by Mitchell et al. comprises two blackening combinations. The first combination includes a compound containing ionizable chloride to etch the surface and a salt or acid of molybdenum to deposit black molybdic oxide. The second combination includes a metal salt and a sulfur-bearing compound which react together to form a black or grey reaction product. Following the teachings of Mitchell in the above patent, a coating has been deposited which comprises a mixture of molybdic oxide and a metal sulfide. The coating is black and adherent as claimed, but measurements of solar properties reveal a high solar energy absorptance and a concomitantly high thermal emmisivity, making the coated surface inefficient for solar energy collection.
The present invention provides a rapid, simple and inexpensive electroless method for producing a solar energy absorbing coating having high solar energy absorptance and the desired low thermal emissitivy.